There are a range of circumstances in which it is desirable to obtain an electrical measurement of a compound action potential (CAP) evoked on a neural pathway by an electrical stimulus applied to the neural pathway. However, this can be a difficult task as an observed CAP signal will typically have a maximum amplitude in the range of microvolts, whereas a stimulus applied to evoke the CAP is typically several volts. Electrode artefact usually results from the stimulus, and manifests as a decaying output of several millivolts throughout the time that the CAP occurs, presenting a significant obstacle to isolating the CAP of interest. As the neural response can be contemporaneous with the stimulus and/or the stimulus artefact, CAP measurements present a difficult challenge of amplifier design. In practice, many non-ideal aspects of a circuit lead to artefact, and as these mostly have a decaying exponential characteristic which can be of either positive or negative polarity, identification and elimination of sources of artefact can be laborious.
A number of approaches have been proposed for recording a CAP, including those of King (U.S. Pat. No. 5,913,882), Nygard (U.S. Pat. No. 5,758,651) and Daly (US Patent Application No. 2007/0225767).
Evoked responses are less difficult to detect when they appear later in time than the artifact, or when the signal-to-noise ratio is sufficiently high. The artifact is often restricted to a time of 1-2 ms after the stimulus and so, provided the neural response is detected after this time window, data can be obtained. This is the case in surgical monitoring where there are large distances between the stimulating and recording electrodes so that the propagation time from the stimulus site to the recording electrodes exceeds 2 ms. However, to characterize the responses from the dorsal columns for example, high stimulation currents and close proximity between electrodes are required, and therefore the measurement process must overcome contemporaneous artifact directly. Similar considerations can arise in deep brain stimulation where it can be desirable to stimulate a neural structure and immediately measure the response of that structure before the neural response propagates elsewhere.
Implanted electrical stimulus devices must also provide for charge recovery in order to ensure that transient currents delivered by stimuli do not lead to a net DC injection of charge into the tissue. One approach is to provide capacitors in series on each electrode, to prevent DC transfer to tissue, and such capacitors are often a requirement of regulatory bodies in order for an active implantable device to obtain market approval. Another arrangement as shown in FIG. 1 omits electrode capacitors, and instead provides switches to short circuit the stimulus and sense electrodes e1-e4 to each other to effect charge recovery between stimuli, and also provides a star network of resistors each of a value in the range of perhaps hundreds of kΩ, permanently connecting all electrodes together in order to equilibrate charge before the device is powered on, as shown in FIG. 1. However, the provision of electrode capacitors or a star network of resistors between the electrodes and the measurement amplifier can give rise to considerable effects of artefact which can interfere with attempts to measure small CAP signals.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
In this specification, a statement that an element may be “at least one of” a list of options is to be understood that the element may be any one of the listed options, or may be any combination of two or more of the listed options.